1. Field of the Invention
The present invention relates to a compact electromechanical vibrator and to an implementation method.
2. Description of the Prior Art
Electromechanical vibrators find applications notably within the scope of seismic exploration operations where seismic images of an underground formation to be explored are formed from elastic waves picked up by suitable seismic receivers, these waves being reflected by the subsoil discontinuities in response to waves emitted by a source, either an impulsive source: charge of explosives in a hole, air guns towed by a boat at sea, etc., or vibrators emitting variable-duration signals, generally with a variable frequency. The frequency variation can be continuous within a certain frequency range (sweep) as described in U.S. Pat. No. 2,688,124 or discontinuous with binary coding as in French patent 2,589,587.
The vibrators can be electromagnetic, electro-hydraulic or piezoelectric type vibrators. A piezoelectric type vibrator generally comprises a baseplate coupled with the ground, a relatively heavy inertia mass coupled with the baseplate by means of one or more piezoelectric transducers. Each transducer comprises for example a bar consisting of a single piece or of a pile of piezoelectric ceramic disks coupled in series, and it is connected to a frequency or phase modulated vibratory signal generator. A piezoelectric vibrator is for example described in U.S. Pat. No. 5,005,665.
In order to obtain sufficient force and vibrational amplitude, it is necessary to use ceramic pillars of relatively great length, of the order of 1 m in practice. As a result of this piling, the heavy inertia mass is placed relatively high so that the vibrator is unstable and ill-suited for working on an even slightly sloping ground without previous anchoring. Furthermore, ceramic pillars of great length prove to be more fragile.
The electromechanical vibrator according to the invention allows application of a vibrational force to a material medium while avoiding the aforementioned drawbacks. It comprises a baseplate intended to be coupled in operation with a surface of this medium, an inertia mass tightly connected to the baseplate by a vibrator electrically connected to an excitation generator.
The vibrator comprises at least two electromechanical transducers including each at least one elongate motive element (of any type, but preferably comprising one pillar per pile of disks made of a piezoelectric or magnetostrictive material), each motive element of the first transducer tightly connecting the baseplate to a relay plate and each motive element of the second transducer being tightly connected, at one end, to the relay plate and, at the opposite end, to the inertia mass placed between the baseplate and the relay plate, the excitation generator comprising a phase shifter for applying phase-shifted signals to the two transducers respectively.
According to an embodiment, the vibrator comprises a pair of transducers placed on the same side of a relay plate and tightly coupled thereto, each motive element of the second transducer tightly connecting the relay plate to the inertia mass.
According to another embodiment, the vibrator comprises at least two pairs of transducers, the transducers of each pair being placed on the same side of a relay plate and tightly coupled thereto, each motive element of the second transducer being tightly connected, at one end, to the relay plate and connected, at the opposite end, to the inertia mass by means of the second pair of transducers and of relay plates.
According, to a preferred embodiment, the electromechanical transducers each comprise one or more pillars made of a piezoelectric or magnetostrictive material.
With such a layout of at least one pair of electromechanical transducers with motive elements of limited length, coupled and combined by means of one or more relay plates, the inertia mass can be positioned next to the baseplate, which allows obtaining of a very robust and very stable vibrator of limited overall height.
Each transducer preferably comprises several elongate motive elements arranged parallel to each other and connected in parallel to the phase shifter.
According to a preferred embodiment, the size of the vibrator is decreased by making one or more recesses through the inertia mass through which run the motive elements of each transducer.
According to another preferred embodiment, at least one housing is provided in the inertia mass so that part of the length of the motive elements of the second transducer is housed in the thickness thereof.
The excitation generator preferably comprises a connection allowing separate excitation of the two transducers of each pair with amplitude and phase modulable excitation signals.
In applications where the stability criterion is not particularly sought, it is possible to use a vibrator comprising at least three electromechanical transducers interconnecting the baseplate and the inertia mass, these transducers being connected to each other by means of at least two relay plates, the excitation generator being connected to the various transducers by a phase shifter suited to apply phase-shifted excitation signals thereto respectively, so as to combine the vibrational signals emitted by the various transducers.
In certain applications where the vibrator is to be placed in a cavity extended by a reservoir exploitation well, the baseplate, the inertia mass and each relay plate can be provided with a central recess for passage of tubing.
The invention also relates to a method of seismic monitoring of an underground zone such as an underground fluid storage reservoir for example, comprising:
coupling an elastic wave receiver with the zone (in a well for example),
installing, for the zone monitoring period, at least one vibrator comprising a baseplate intended to be coupled in operation with a surface of the zone (the bottom of a cavity bored in the ground for example or the lateral walls thereof), an inertia mass tightly connected to the baseplate by at least two electromechanical transducers arranged on the same side of a relay plate, and tightly connected thereto, each transducer comprising at least one elongate motive element electrically connected to an excitation generator by the phase shifter suited to apply phase-shifted signals thereto respectively so as to combine the vibrational signals emitted by the various transducers,
installing a network of electric cables connected to the transducers of each vibrator, and
connecting the cables of this network to a central station comprising at least one electric excitation generator allowing excitation of each vibrator by amplitude and phase modulable signals, a selective connector of each vibrator by means of the network of electric cables, and a recorder for recording the signals coming from the underground zone in response to the elastic waves selectively transmitted in the ground by each vibrator.
The vibrators are preferably placed in cavities provided in the ground and the elastic wave receivers coupled with the formation in one or more wells drilled in the formation, whose locations are distinct from the cavities or not. They are suitably coupled with the cavity so as to generate longitudinal or transverse waves in the formation.
For seismic monitoring of an underground zone, a plurality of assemblies made up of vibrators and receivers can be provided and monitoring cycles can be carried out with excitation of the various vibrators (successively or simultaneously according to particular modes) and recording of the waves reflected by the formations in response to the emitted waves received by various receivers.
The vibrator according to the invention can be used within the scope of onshore seismic prospecting operations as well as seismic monitoring operations carried out onshore or at the bottom of a water mass.
As a result of the robustness and compactness thereof, the vibrator according to the invention can be readily installed permanently in a cavity provided in the ground and intermittently excited by means of the network, within the scope of periodic underground zone monitoring operations, or it can be used for seismic prospecting operations.